G to A hypermutation of TT virus.

APOBEC3 proteins function as part of innate antiviral immunity and induce G to A hypermutation in retroviruses and hepatitis B virus (HBV) genomes. Whether APOBEC3 proteins affect viruses that replicate without a reverse transcription step is unknown. TT virus (TTV), known to present in serum of healthy individuals and HBV carriers, has a single-stranded circular DNA genome and replicates without reverse transcription.